Tangible Equity Units - Additional Information (Detail) shares in Millions, $ in Millions	1 Months Ended	3 Months Ended
	Jun. 30, 2015 USD ($) $ / unit shares	Mar. 31, 2018 unit $ / unit
TEUs - senior amortizing notes due 2018 [Member]
Tangible Equity Units [Line Items]
Principal amount per amortizing note | $ / unit		10.95
Debt instrument, stated interest rate		1.50%
Debt instrument, maturity date		Jun. 07, 2018
7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Number of tangible equity units issued | shares	9.2
Tangible equity unit rate	7.50%
Stated amount per tangible equity unit | $ / unit	50.00
Total net proceeds | $	$ 445
Equity Component of 7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Purchase contract, mandatory settlement date		Jun. 07, 2018
Less Than Reference Price [Member] | Equity Component of 7.50% Tangible Equity Units [Member] | Settled in WGP Common Units [Member]
Tangible Equity Units [Line Items]
Maximum settlement rate | unit		0.8862
Exceeds Threshold Appreciation Price [Member] | Equity Component of 7.50% Tangible Equity Units [Member] | Settled in WGP Common Units [Member]
Tangible Equity Units [Line Items]
Minimum settlement rate | unit		0.7383